Bad reactions

Wednesday, May 7, 2008

With French and German companies lining up to build new nuclear power stations in Britain, the die now seems cast for nuclear. Or is it?

The government's goal is certainly ambitious. Ten countries - primarily the UK, US, France and Canada, but also including Japan, Korea, Brazil, Argentina, South Africa and Switzerland - have set up the Generation IV International Forum. It will develop a successor nuclear energy system to the previous Generations I (Magnox) and II (advanced gas-cooled reactors and the Sizewell B light water reactor) and follow the Generation III systems now being built. The latter includes the French Areva evolutionary pressure reactor (EPR), the prototype of which is being constructed at Olkiluoto in Finland, with another being built in France.

Improved versions
It is intended that these Generation III models, plus (hopefully) improved versions in future, will lead reactor orders through to 2030, after which it is hoped that Generation IV will kick in, with the goal of nuclear sustainability.

However, the roadmap to get there is beset by practical problems that may prove insurmountable. Generation II and III nuclear plants operate in a "once-through" mode, which means that only half the 0.7% fissionable uranium 235 content of natural uranium goes into the fuel, while most of the heavy metal ends up in enrichment tails and in spent fuel as waste. This, therefore, requires a constant and increasing supply of natural uranium to meet the rising demand for electricity, while intensifying the already unresolved problem of what to do with vast accumulations of radioactive waste.

Even the International Atomic Energy Agency and the optimistic Organisation for Economic Cooperation and Development put the total world uranium reserves at 4.7m tonnes, and that assumes a purchase price of at least $130/kg. In fact, prices are currently nearly twice as high, yet primary uranium production is falling. But even if the figures were roughly correct and not significantly inflated, the total of known uranium resources is expected to be exhausted by 2030. If fast reactors were to be introduced by then, which is the centrepiece of the strategy, a further 10m tonnes - twice the known resources - would have to be ready for production, and this could only come from "speculative and undiscovered resources".

The nuclear power industry answers this by referring to the universality of uranium in the Earth's crust and in sea water. But the enormous energy needed to extract it from these low-concentration sources would exceed the energy output of the fission of the fuel provided.

These pressures are already being felt. The US gets half its nuclear fuel from diluted former nuclear weapons' highly-enriched uranium from Russia. And even Russia, with insufficient primary production, will be forced to rely on ex-weapons material to power its planned expansion. The UK's aim to secure energy supplies will not be aided by importing 100% of nuclear fuels, and that's on top of increased dependence on imported fossil fuels, notably gas.

Meanwhile, Japan has closed seven nuclear power stations built on an earthquake fault line. The Olkiluoto reactor is already two years behind schedule after just two years' building and has a £1bn cost overrun so far, and there can be no reliable evidence on the economics of nuclear power until the new designs of the Westinghouse AP1000 and European EPR water reactors have been fully tested over many years in service. Contrary to claims by the industry, unresolved questions of cost and the looming shortage of uranium are the biggest challenges to the nuclear revival.

To overcome the fragility of this recovery, the industry looks to Generation IV development of the fast reactor by 2030 as the key to ultimate nuclear sustainability. However, if for this purpose the fast reactor were adopted in "breeder" mode, an even greater quantity of highly radioactive actinoids (plutonium, neptunium, americium and curium) would be generated, exacerbating still further the waste management problem. If, on the other hand, the fast reactor were adopted in "burner" mode, as currently seems likely to prevail, the waste problem is alleviated, but there is no sustainability.

The Generation IV fuel systems offer at present six types, of which two are emerging as likely candidates. One is the very high temperature thermal reactor (VHTR), which can be used for coal gasification as well as thermo-chemical hydrogen production. The US government favours this because a hydrogen economy is seen as the solution to the exhaustion of oil reserves, and the petrol derived from it.

The main problem with VHTR, which has a coolant system outlet temperature of about 1,000C, is likely to arise from irradiation characterised by the Wigner effect - the displacement of atoms in a solid caused by neutron radiation - and from progressive disintegration by neutron bombardment. Indeed, a similar problem with the Wigner energy in Pile 1 at Windscale (now Sellafield) caused the fire in 1957 and melted the fuel elements. Given the very high temperatures needed for this complex and quite likely unstable process, its viability would need rigorous and exhaustive testing before such a problematic reactor were ever adopted.

Repetitive cycle
The second favoured Generation IV candidate is the sodium-cooled fast reactor system (SFR). The idea here is that as the supply of natural uranium declines, it is replaced by a plutonium-based fuel that is incrementally augmented by fresh plutonium in a repetitive cycle, providing claims of sustainability. It is envisaged that there is a gain in the plutonium in a surrounding "blanket" of uranium 238 over and above the plutonium consumed in the reaction, with a doubling time of 15 to 20 years.

Again there are two key problems. It is a burner reactor, not a breeder, so that while reducing waste management problems, it does not provide for sustainability. Second, even if fast reactors of this kind could be successfully deployed - a big if - the doubling time of 15 to 20 years would require supplies of natural uranium to be maintained for decades, if not centuries, until the fleet of "once-through" reactors can be progressively replaced. And the uranium simply is not available for that timespan.